In the formation of integrated circuits, the components of the integrated circuit devices need to be patterned to form desirable shapes. A typical patterning process includes a lithography process, which includes coating a photo resist over a target layer that is to be patterned, exposing the photo resist using a lithography mask, developing the photo resist, and using the developed photo resist to etch the target layer. As a result, the layout of the developed photo resist is transferred to the underlying layer. The photo resist is then removed.
There are some integrated circuits whose formation involves high topology features. In the high topology features, the height difference between a high feature and a neighboring low feature may be as high as about 100 nm to about 300 nm, or even higher. For example, Fin Field-Effect Transistors (FinFETs) typically have fins much higher than the top surface of surrounding Shallow Trench Isolation (STI) regions. In addition, gate dielectrics, polysilicon gates, and hard masks may be stacked over the fins. Therefore, the heights of these layers are further added to the heights of the fins. This results in a severe topography problem that incurs difficulty in the respective lithography process.
When lithography processes are performed on the wafers having a high topology, the photo resist and the Bottom Anti-Reflective Coating (BARC) need to be formed. The photo resist and the BARC suffer from thickness non-uniformity. For example, the BARC may be accumulated at the corner formed between an edge of a tall feature and a top surface of a low feature. On the other hand, the portion of the BARC directly over the tall feature may have a very small thickness. As a result, the BARC thickness is not uniform throughout a respective wafer.
To reduce the thickness uniformity, various approaches were taken. For example, a negative tone photo resist may be used to fill the gaps next to tall features. A positive tone photo resist is then used to define the desirable patterns. This process, however, incurs increased process cost since an additional negative tone photo resist needs to be formed, and an additional lithography process and lithography mask are needed to pattern the negative tone photo resist. In addition, in these processes, damages may occur to the portions of the tall features that are not supposed to be etched, while residues of the low features may be undesirable left after the patterning.